What does {'INTERFACE'} mean when returned from `invoke`, `modal` or `execute`?
The 'INTERFACE' operator return value is used to indicate where an operator has successfully completed by opening a menu or other UI component.

The comment in the source code for the operator return types states that it is:

/* "used for operators that act indirectly (eg. popup menu)" */
/* "note: this isn't great design (using operators to trigger UI) avoid where possible." */



  Source: source/blender/makesdna/DNA_windowmanager_types.h


This new return type was added during Commit - Fix T41041: "'Delete keyframe' removes markers too"

Reason for commit update was: 


  "Operators that trigger UI events (but nothing else)
  were using 'CANCELLED' making it impossible to tell if an invoke
  function failed, or opened a menu."